The lift-off technique is known as means for forming metal wiring patterns of aluminum, copper, tantalum or the like on semiconductor substrates. The lift-off technique involves, for example, the steps of coating a resist composition on a substrate, exposing the resist film to light through a mask, developing to form a resist pattern on the substrate, depositing a metal film on the resist pattern and the substrate by sputtering or evaporation, and stripping the resist pattern and the metal film thereon together, thus leaving a metal wiring on the substrate. The resist pattern used in the lift-off technique is desired to have a profile having undercuts, also known as “microgrooves,” at the bottom of the resist pattern where the resist contacts the substrate.
Prior art positive lift-off resist compositions known to form patterns of such profile are described in JP-A H08-69111 and JP-A H10-97066. These compositions use different components, but are common in that aromatic hydroxy compounds are used in order to form undercuts. This is because aromatic hydroxy compounds have an adequate alkali dissolution rate to form undercuts near the substrate and control the size thereof. Also JP-A 2001-235872 discloses means for improving the substrate adhesion of a resist. This resist material, however, suffers a film loss phenomenon that as the undercut depth increases, the resist film in the unexposed area is also developed during the development step, becoming thinner. It is difficult to provide the desired undercut depth and maintain the resist film thickness at the same time. It is desired to have a positive lift-off resist composition which ensures a sufficient undercut depth while controlling a film loss during development.
JP-A 2004-94229 discloses a positive photoresist composition comprising a polymer in the form of a novolac resin having a weight average molecular weight of 1,000 to 30,000 versus polystyrene standards in which 3 to 27 mol % of the hydroxyl group hydrogens are substituted by 1,2-naphthoquinonediazidosulfonyl groups, and an alkali-soluble cellulose resin. This composition sometimes fails to form undercuts near the substrate.